1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer. Particularly, the present invention relates to an image forming apparatus including a rotary fixing element having a surface coated with a release agent, and to a transfer medium carrying member for use in such an image forming apparatus.
2. Related Background Art
Hitherto, various multi-color (full-color) image forming apparatuses have been proposed. A typical example of such apparatuses is a full-color electrophotographic copying machine having a developing device of the so-called rotary-type, as shown in FIG. 5.
The full-color electrophotographic copying machine will be briefly described. Referring to FIG. 5, the copying machine includes an image carrying element, comprising a photo-sensitive drum 3, rotatably supported in the apparatus in such a manner as to be rotatable in the direction a indicated by the arrow a in FIG. 5, and image forming means arranged in the outer periphery of the photo-sensitive drum 3. The image forming means may comprise any suitable constituents. In the illustrated example, the image forming means comprises a primary charger 4 for uniformly charging the surface of the photo-sensitive drum 3, an exposure means 8, such as a laser beam exposure device, for radiating a color-separated light image or a light image corresponding thereto and thus forming an electrostatic latent image on the photo-sensitive drum 3, and a rotary developing device 1 for visualizing the electrostatic latent image on the photo-sensitive drum 3.
The rotary developing device 1 includes four developing units 1Y, 1M, 1C and 1BK individually containing developers of four colors, that is, yellow (Y), magenta (M), cyan (C) and black (BK), and a substantially cylindrical casing which is rotatably supported in the apparatus and to which the four developing units 1Y, 1M, 1C and 1BK are mounted. The rotary developing device 1 is constructed such that rotation of the casing causes a desired developing unit to be moved to a position facing the outer peripheral surface of the photo-sensitive drum 3 so as to develop the electrostatic latent image on the drum 3 into a visible image of four colors (full-color development).
The visible image, that is, the toner image, on the photo-sensitive drum 3 is transferred to a transfer medium P (such as paper) being fed as carried by a transfer medium carrying element, comprising a transfer drum 9, of a transfer device, the drum 9 being rotatably supported in the apparatus for rotation in the direction b indicated by the arrow b in FIG. 5. As shown in FIG. 6, the transfer drum 9 includes a pair of cylinder portions 9a and 9b at either end of the drum 9, and a link portion 9c linking together both cylinder portions 9a and 9b. A transfer medium carrying member 93 extends between the non-linked regions of the respective circumferences of the cylinder portions 9a and 9b. The transfer medium carrying member 93 normally comprises a film-shaped dielectric sheet consisting of a film of a polyethylene terephthalate or polyvinylidene fluoride resin. The link portion 9c has a transfer medium gripper 7 for gripping transfer medium P fed from a transfer medium feed device. As shown in FIG. 5, the transfer device includes a transfer discharger 10 serving as the transferring means, and an inner charge-removal discharger 13, which are disposed on the inside of the transfer drum 9. The inner charge-removal discharger 13 constitutes charge removal means together with an outer charge-removal discharger 11 on the outside of the transfer drum 9.
A sequence of processes by which the full-color electrophotographic copying machine having the above construction forms a full-color image will be briefly described.
The primary charger 4 uniformly charges the surface of the photo-sensitive drum 3. Then, the exposure means 8 radiates a light image E corresponding to image information onto the surface, thereby forming an electrostatic latent image on the photo-sensitive drum 3. The rotary developing device 1 develops the electrostatic latent image with a toner containing a resin as a base material and having an average particle size of 12 .mu.m, whereby the latent image is visualized as a toner image on the photo-sensitive drum 3.
Alternatively, a transfer medium P is fed by a resist roller 6 to the transfer drum 9 in synchronization with the formation of the image. When the leading end of the transfer medium P is gripped by the gripper 7, the medium is carried by the transfer drum 9 in the direction b.
In a region where the transfer drum 9 contacts the photo-sensitive drum 3, the transfer discharger 10 effects corona discharge with the opposite polarity to the toner from the reverse side of the transfer medium carrying member, that is, the reverse side of the dielectric sheet 93 of the transfer drum 9, so that the toner image on the photo-sensitive drum 3 is transferred to the surface of the transfer medium P.
After the transfer medium P has been subjected to a required number of transfer processes, the resultant transfer medium P is, while being subjected to charge removal by the charge-removal dischargers 11 and 13, separated from the transfer drum 9 by a separation pawl 15. The transfer medium P is then fed to a fixing device 17 by a conveyance belt 16. After the medium P has been subjected to fixing by heat by the fixing device 17, the resultant medium P is discharged to the outside of the apparatus.
Alternatively, the surface of the photo-sensitive drum 3 is cleaned by a cleaner 12, whereby toner remaining on the surface is removed. Thereafter, the surface of the drum 3 is used in subsequent image forming processes.
The surface of the dielectric sheet 93 of the transfer sheet 9 is cleaned by another cleaner 18 comprising a fur brush or the like and cooperating with a cleaning aid means 19. Thereafter, the surface of the dielectric sheet 93 is used in subsequent image forming processes.
The fixing device 17 employs a roller fixing method in which fixing is effected as a transfer medium having a toner image is clamped and fed by a pair of rollers, and thus pressure is applied. The fixing device 17 particularly employs a heat roller fixing method using heated rollers. In a heat roller fixing method, in order to prevent toner from offsetting, a release agent, such as a silicone oil, is coated on the fixing roller. Particularly in the case of a full-color image forming apparatus in which toner is deposited in a plurality of layers, a relatively great amount of silicone oil is coated since toner is more likely to offset.
Another example of a transfer medium which may be used is a resin film such as a transparent resin film.
Such a conventional image forming apparatus entails the following problems. When a toner image on a transfer medium is fixed by a fixing roller coated with a silicone oil or the like, the transfer medium passed through the rollers may be wet or sticky with oil, etc. This is particularly disadvantageous in the case of two-side image formation (e.g., two-side copying) in which image formation is performed in two stages, the second stage comprising forming an image on the reverse (second) surface of a transfer medium whose front (first) surface already has a fixed toner image. In this case, the second stage has a risk of oil adhering to the transfer medium being transferred and adhered to the photo-sensitive drum 3, thereby contaminating images to be formed by the subsequent image formation.
Specifically, in two-side image formation, a transfer medium, which has been subjected to fixing once, is again fed to the transfer drum 9 by the resist roller 6 in synchronization with the formation of the relevant image. Then, the transfer medium has its leading end gripped by the gripper 7, and is carried by the transfer drum 9 in the direction b shown in FIG. 5 with the first surface of the medium contacting the transfer medium carrying member, that is, the dielectric sheet 93, so that the second surface of the transfer medium is subjected to transfer processes similar to those performed with respect to the first surface. Thereafter, the resultant transfer medium is separated from the transfer drum 9, and again sent to the fixing device 17, in which fixing is performed to obtain a medium having images on both sides thereof (two-sided copy). However, when the transfer medium is separated from the transfer drum 9 after the completion of the transfer processes of the second stage, oil or other release agent is transferred from the first surface of the transfer medium to the outer surface of the dielectric sheet 93, and is adhered thereto. As a result, the oil may transfer and adhere to the surface of the photo-sensitive drum 3 during mutually-contacting rotation of the transfer and photo-sensitive drums 9 and 3. Thus, there is a risk of the photoconductor of the drum 3 being contaminated. In such a case, in the subsequent image forming sequence, the oil on the photoconductor may adsorb remaining toner or toner for development, and thus may be transferred to a transfer medium, thereby causing image contamination or fogging.
In a conventional image forming apparatus, such as above, neither the cleaner 12 nor the cleaner 18 comprising an elastic blade, etc. is able to remove oil adhering to the photo-sensitive drum 3 or the dielectric sheet 93. Consequently, once oil has adhered to a photo-sensitive drum or dielectric sheet, it has to be replaced with a new member. Thus, the above problems inevitably increase costs and decrease serviceability.
The conventional practice directed to coping with these problems includes decreasing the amount of silicone oil or other release agent coated on the fixing roller so as to decrease the amount of oil, etc. which can adhere to the first surface of a transfer medium. With this method, it is possible to prevent oil, etc. from being transferred and adhered to the photo-sensitive drum during a single execution of the two-side image formation sequence. However, when the sequence is performed a plurality of times, oil may deposit by small amounts on the dielectric sheet of the transfer drum, thereby involving the risk of oil being transferred and adhered to the photo-sensitive drum.